Method of providing a protective coating composition for molten aluminum and alkali metal environments

ABSTRACT

The invention is directed to a method of providing a protective coating composition that protects a refractory wall or lining from chemical attack by molten aluminum and molten alkali metals. The method includes the steps of coating a refractory wall or liner with an aqueous protective composition that includes, by weight of the solids, about 20-90% Al 2 O 3  (excluding calcined alumina), about 15-55% SiO 2  and about 1-15% of a metallic non-wetting agent; and evaporating the water before contacting the protective coating with the reactive molten metal.

RELATED APPLICATIONS

This patent application is a continuation-in-part of U.S. patentapplication Ser. No. 15/235,774, the disclosure of which is incorporatedby reference.

FIELD OF THE INVENTION

This invention is directed to a method of providing a protective coatingcomposition that can be easily applied to molten metal containmentvessels by brushing, rolling, spraying or the like. The compositionprotects the vessel walls against corrosion and other degradation causedby exposure to molten aluminum and alkali metals and vapors contained inthe vessel.

BACKGROUND OF THE INVENTION

Molten aluminum and molten alkali metals and vapors are known to attackrefractory walls and linings made of alumina, silica, magnesia,magnesite, chromite, and other materials. Prolonged exposure to thesemolten metals and vapors promotes corrosion and degradation of variousparts of furnaces and other containment vessels, including walls,ceilings, roofs, exhaust ducts, floors, ramps, skim shelves, spouts, tapholes, troughs, runners, launders, lentils, door jams, and doors. In thepast, this required periodic replacement of the refractory liningsand/or the containment vessels. This periodic replacement often entailedsignificant down time and expense.

Only a small amount of corrosion and degradation is required toadversely affect the thermal insulation properties of the refractorywall or lining. When the refractory wall or lining is formed ofalumina-silicate, as is commonly the case, the molten aluminumchemically reacts with the refractory to form corundum, which is amixture of Al₂O₃ with unreacted Si and Al. This reaction productstrongly attaches itself to the refractory by filling its porosity, andis very difficult to remove.

By way of example, molten aluminum reacts with silica in the refractorywall or lining as follows:

4Al+3SiO₂→2Al₂O₃+3Si

If the molten aluminum is a metal-bearing alloy, such as analuminum-magnesium alloy, the following additional reactions may occur:

2Mg+SiO₂→2MgO+Si

3Mg+4Al₂O₃→3MgAl₂O₄+2Al

Other oxides frequently found in refractories are also reduced byreaction with molten aluminum and alkali metals and vapor. These includeoxides of titanium and iron, for example. There is a need or desire foran easy-to-apply coating that protects refractory walls and linings fromchemical attack by molten aluminum and alkali metals and vapors.

SUMMARY OF THE INVENTION

The present invention is directed to a method of providing an aqueousprotective coating composition that can be easily applied to any ceramicor masonry surface by brushing, rolling, spraying or the like, andsubsequently dried. The invention is also directed to a method ofproviding the dried coating composition. The dried coating compositionprotects the ceramic or masonry surface from chemical attack from moltenaluminum and/or alkali metals and vapors.

The method includes the steps of a) combining and mixing a quantity ofsynthetic or natural gum with a quantity of water to provide a firstmixture, b) combining and mixing the first mixture with an aqueouscolloidal silica dispersion to provide a second mixture; and c) addingmullite, calcined alumina and a non-metallic wetting agent to the secondmixture, and mixing the ingredients together to form the aqueousprotective coating composition. The composition is then applied to asubstrate and dried to provide the protective coating.

The dried protective coating composition includes the followingingredients:

-   -   about 20% to about 90% by weight Al₂O₃;    -   about 15% to about 55% by weight SiO₂; and    -   about 1% to about 40% by weight of a metallic non-wetting agent;        wherein the Al₂O₃, SiO₂, and non-wetting agent together        constitute at least about 90% by weight of the protective        coating composition.

The aqueous coating composition includes about 5% to about 40% by weightwater and about 60% to about 95% by weight solids. The solids include,on a dry weight basis:

-   -   about 20% to about 90% by weight Al₃O₃ (excluding calcined        alumina);    -   about 15% to about 55% by weight SiO₂; and    -   about 1% to about 15% by weight of a metallic non-wetting agent;    -   wherein the Al₂O₃, SiO₂, and non-wetting agent together        constitute at least about 90% by weight of the solids.

Some of the Al₂O₃ and SiO₂ are suitably in the form of mullite, havingthe chemical formula 3Al₂O₃·2SiO₂. Some of the silica is suitably addedwith the water in the form of colloidal silica. Some of the alumina canbe in the form of calcined alumina. The non-wetting agent is believed tocombine with the Al₂O₃ and SiO₂ to form a combination which resistspenetration and reaction with molten aluminum and alkali metals andvapors, thereby providing the protective coating.

With the foregoing in mind, it is a feature and advantage of theinvention to provide a protective coating composition for moltenaluminum and alkali metal environments that can be easily applied andre-applied to a ceramic or masonry surface by brushing, spraying,rolling or similar techniques.

It is also a feature and advantage of the invention to provide anenvironmentally safe protective coating composition that issubstantially free of organic components.

It is also a feature and advantage of the invention to provide aprotective coating composition for molten aluminum and alkali metalenvironments that is relatively inexpensive to formulate, purchase,apply to a substrate, and use.

These and other features and advantages of the invention will becomefurther apparent from the following detailed description of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to a method of providing an aqueous protectivecoating composition that can be easily applied to a concrete or masonrysurface by brushing, spraying, rolling or the like and subsequentlydried. The invention is also directed to a method of providing the driedprotective coating composition that provides effective protection inmolten aluminum and alkali metal environments.

The dried protective coating composition comprises about 20% to about90% by weight Al₂O₃. Suitably, the dried protective coating compositionincludes about 30% to about 70% by weight Al₂O₃, or about 40% to about60% by weight Al₂O₃. The dried protective coating composition comprisesabout 15% to about 55% by weight SiO₂, suitably about 25% to about 50%by weight SiO₂, or about 30% to about 45% by weight SiO₂.

Some or all of the Al₂O₃ and some of the SiO₂ can be provided as mullitehaving the chemical formula 3Al₂SO₃·2SiO₂. The mullite can be micronizedto a median particle diameter of about 1-100 microns, suitably about2-10 microns. One suitable commercially available mullite is MJ5Mmicronized mullite, available from the Kyanite Mining Corporation inDillwyn, Virginia. MJ5M micronized mullite has a median particlediameter of about 4-9 microns and contains about 55-60% by weight Al₂O₃,about 38-43% by weight SiO₂, less than about 1% by weight Fe₂O₃, about1-2% by weight TiO₂, and less than about 1% by weight alkali andalkaline earth metal oxides (e.g., CaO, MgO, Na₂O, K₂O). The mullite mayconstitute about 30% to about 70% by weight of the dried protectivecoating composition, suitably about 40% to about 60% by weight. Themicronized mullite primarily benefits the stability of the wet mixture,helping to keep the ingredients in suspension instead of settling out.

Some or all of the Al₂O₃ and some of the SiO₂ can be provided as mullitehaving the chemical formula 3Al₂SO₃·2SiO₂. The mullite can be micronizedto a median particle diameter of about 1-100 microns, suitably about2-10 microns. One suitable commercially available mullite is MJ5Mmicronized Mullite, available from the Kyanite Mining Corporation inDillwyn, Va. MJ5M Micronized Mullite has a median particle diameter ofabout 4 to about 9 microns and contains about 55-60% by weight Al₂O₃,about 38-43% by weight SiO₂, less than about 1% by weight Fe₂O₃, about1-2% by weight TiO₂, and less than about 1% by weight alkali andalkaline earth metal oxides (e.g., CaO, MgO, Na₂O, K₂O). The mullite mayconstitute about 30% to about 70% by weight of the dried protectivecoating composition, suitably about 40% to about 60% by weight. The useof micronized mullite improves the particle packing and the stability ofthe dried protective coating composition.

As an alternative to MJ5M micronized mullite, it has been foundadvantageous to use a mixture of micronized mullites having smaller andlarger particle sizes. The micronized mullite mixture has the advantageof being able to reduce the porosity of the protective coatingcomposition by filling pores and voids that would otherwise be leftopen. In one embodiment, 325-mesh micronized mullite (325M) having alarger median particle diameter of about 10-15 microns can be mixed withMJ5M micronized mullite having a smaller median particle diameter ofabout 4-9 microns. One suitable micronized mullite mixture containsabout 10 to about 50% by weight of the smaller micronized mullite andabout 50 to about 90% by weight of the larger micronized mullite, orabout 20 to about 30% by weight of the smaller micronized mullite andabout 70 to about 80% by weight of the larger micronized mullite.

In one embodiment, micronized mullite having the following particle sizedistributions can be comined in any of the above weight ratios, or aweight ratio of about 25% smaller micronized mullite and about 75%larger micronized mullite.

Smaller Micronized Mullite (MJ5M) d10  1.28 microns d50  6.83 micronsd90 21.72 microns Larger Micronized Mullite (325M) d10  1.58 microns d5011.43 microns d90 43.87 microns

In the above example, the designation “d50” means that 50% of thesmaller micronized mullite had a diameter of 6.83 microns or less, and50% of the larger micronized mullite particles had a diameter of 11.43microns or less. The designators “d10” and “d90” have correspondingpercentage-based meanings. This combination of smaller and largermicronized mullite particles has been shown to provide some reduction inporosity compared to using either the smaller or the larger micronizedmullite alone.

Some of the Al₂O₃ can be calcined alumina, suitably having a medianparticle size of about 1 to about 50 microns, or about 2 to about 25microns, or about 3 to about 10 microns. When calcined alumina ispresent, the dried protective coating composition may include about 5%to about 40% by weight calcined alumina, or about 15% to about 35% byweight calcined alumina, or about 20% to about 30% by weight calcinedalumina. One suitable calcined alumina is sold under the name AC2-325M,available from AluChem, Inc. of Reading, Ohio. Calcined alumina can bepurchased or made by calcining aluminum powder at 1200-1300° C. toconvert it to pure Al₂O₃.

Some of the SiO₂ can be colloidal silica having a median particlediameter of about 1-100 nanometers, suitably about 4-100 nanometers, orabout 6-50 nanometers, or about 8-20 nanometers. The colloidal silica isprovided in an aqueous colloidal suspension that includes about 30-60%by weight colloidal silica particles and about 40-70% by weight water,suitably about 40% by weight colloidal silica particles and about 60% byweight water. When the protective coating composition is dried, thecolloidal silica particles act as a binder between the remainingingredients and to the substrate. The colloidal silica particles mayconstitute about 5% to about 30% by weight of the dried protectivecoating composition, suitably about 10% to about 25% by weight.

In one embodiment, the particle sizes of the colloidal silica can beoptimized in order to reduce the porosity of the protective coatingcomposition by filling pores and voids that would otherwise be leftopen. Smaller colloidal silica particles having a median particlediameter of 1 to less than 50 nanometers, or about 5 to about 20nanometers, can be combined with larger colloidal silica particleshaving a median particle diameter of about 50 to about 100 nanometers,or about 60 to about 80 nanometers. Suitable weight ranges are fromabout 10 to about 90% smaller colloidal silica particles and about 10 toabout 90% by weight larger colloidal silica particles, or about 20 toabout 80% by weight smaller colloidal silica particles and about 20 toabout 80%, by weight larger colloidal silica particles.

The dried protective coating composition includes about 1% to about 15%by weight of a metallic non-wetting agent, suitably about 3% to about13% by weight, or about 5% to about 12% by weight. The metallicnon-wetting agent is non-wetting as to molten aluminum and alkali metalsand vapors, and resists wetting by these metals. It is believed that themetallic non-wetting agent combines with the alumina and/or silica inthe protective coating composition to provide the overall compositionwith non-wetting properties and substantial chemical inertness to moltenaluminum (including aluminum-based alloys) and alkali metals and vapors.

A wide variety of metallic non-wetting agents can be employed in theprotective coating composition. Examples of metallic non-wetting agentsinclude metal phosphates such as lanthanum phosphate, trisodiumphosphate, tetrasodium phosphate, sodium pyrophosphate, magnesiumphosphate, potassium phosphate, barium phosphate, iron (III) phosphate,copper (III) phosphate, iron (II) phosphate, calcium phosphate, nickel(II) phosphate, strontium phosphate, aluminum phosphate, aluminumhydrogen phosphate, aluminum dihydrogen phosphate, aluminumpyrophosphate, aluminum perphosphate, aluminum metaphosphate, andcombinations thereof. When heated to temperatures above 980° C. totemperatures characteristic of an aluminum melting furnace, thesephosphates can combine with the alumina to form an aluminumorthophosphate bond, providing the protective coating composition with ahigh degree of resistance to molten metal attack.

Other examples of metallic non-wetting agents include zirconiumsilicates such as zirconium metasilicate (Zr (SiO₃)₂) and zirconiumorthosilicate (ZrSiO₄); Group II metal sulfates such as barium sulfate,strontium sulfate, calcium sulfate and magnesium sulfate; andcombinations thereof. These compounds may decompose at high temperaturesto form oxides which in turn, react with alumina to form stable oxidesphases. For example, barium sulfate decomposes into barium oxide (BaO)at high temperatures, which in turn combines with the alumina to formbarium hexaluminate (BaO·6Al₂O₃) or the monaluminate spinel (BaO·Al₂O₃),both of which are stable and resistant to molten metal attack.

Other examples of metallic non-wetting agents include Group II metalhalides such as barium fluoride, barium chloride, barium bromide,strontium fluoride, strontium chloride, strontium bromide, calciumfluoride, calcium chloride, calcium bromide, magnesium fluoride,magnesium chloride, magnesium bromide, and combinations thereof. Thesecompounds are thermodynamically stable, with relatively high heats offormation, and exhibit non-wetting and de-wetting behavior attemperatures exceeding 1100° C.

Other examples of metallic non-wetting agents include metallic nitridessuch as boron nitride, zirconium nitride, aluminum nitride, siliconnitride, and the like; metallic carbides such as barium carbide,aluminum carbide, tungsten carbide, tungsten-nickel carbide complexes,and the like; and combinations thereof. These compounds arethermodynamically stable at high temperatures, and are non-wetting andresistant to chemical reaction with molten aluminum and alkali-basedmetals.

Other examples of metallic non-wetting agents include compounds andcomplexes of aluminum and/or silicon that combine with the Al₂O₃ and/orSiO₂ to provide reduced wetting and increased resistance to attack bymolten aluminum and alkali metals and vapors. Examples include withoutlimitation magnesium aluminate spinel (MgAl₃O₄); stack structures ofalumina and zirconia such as ZrO₂·Al₂O₃, ZrO₂·Al₂O₃·ZrO₂, andAl₂O₃·ZrO₂·Al₂O₃; aluminum halides such as aluminum fluoride aluminumchloride, aluminum bromide, sodium aluminum tetrafluoride, sodiumaluminum tetrachloride, sodium aluminum tetrabromide, potassium aluminumtetrafluoride, potassium aluminum tetrachloride, and potassium aluminumtetrabromide; calcium aluminate, calcium hexaluminate (C_(a)O·6Al₂O₃),aluminum titanate (Al₂TiO₅), calcium silicate, and combinations thereof.

Other examples of metallic non-wetting agents include Group II metalcarbonates and celsians such as barium carbonate, strontium carbonate,barium celsian, strontium celsian, and combinations thereof. Alsoincluded are fluorides, chlorides and bromides of titanium, zirconium,hafnium, copper and strontium, and combinations thereof.

The Al₂O₃, SiO₂, and non-wetting agent together should constitute atleast about 90% by weight, or at least about 94% by weight, or at leastabout 97% by weight of the dried protective coating composition. Allother ingredients (including impurities, if any) should be kept to aminimum.

The dried protective coating composition may also include about 0.01% toabout 2% by weight, suitably about 0.01% to about 1% by weight of awater-soluble thickening agent. Suitable thickening agents includewithout limitation water-soluble synthetic or natural gums which helpmaintain the solid ingredients in suspension before drying. Suitablegums include without limitation xanthan gum, guar gum, alginates, locustbean gum, and combinations thereof. One suitable gum is xanthan gum soldunder the name KELZAN® by CP Kelco Co. of Atlanta, Ga.

Except for the thickening agent, the dried protective coatingcomposition is either free or substantially free of organic ingredients.When present, the sum of all organic ingredients, inclusive of thethickening agent, is suitably not more than about 8% by weight, or notmore than 5% by weight, or not more than about 2.5% by weight of thedried protective coating composition. The relative absence of organiccomponents promotes a healthy and safe work environment. The protectivecoating compositions should also be free or substantially free ofgelling agents, acidic compounds, alkali hydroxides and other strongbases.

The present invention also includes the aqueous protective coatingcomposition prior to drying. The aqueous protective coating compositioncomprises about 5% to about 40% by weight water, suitably about 10% toabout 30% by weight water, or about 15% to about 25% by weight water.Most or all of the water is combined with colloidal silica in theamounts indicated above, to form an aqueous colloidal dispersion. Theaqueous colloidal dispersion is then mixed with the remainingingredients of the protective coating composition to form an aqueousprotective coating composition including about 60% to about 95% byweight solids, suitably about 70% to about 90% by weight solids, orabout 75% to about 85% by weight solids. Water can be the only liquidcomponent. The aqueous protective coating composition should be free, orsubstantially free, of organic liquids. The term “substantially free”indicates an organic liquid content of not more than about 8% by weight,or not more than about 5% based on the total liquid weight.

The aqueous colloidal silica dispersion and the remaining solidingredients are mixed together to form an aqueous protective coatingcomposition which, excluding the water, contains the same solidingredients in the same amounts as described above for the driedprotective coating composition. The aqueous protective coatingcomposition is then applied to a ceramic or masonry surface usingbrushing, spraying rolling, or another suitable technique. Afterapplication, the aqueous protective coating composition is dried with orwithout heat to provide the dried protective coating having thecomposition described above.

EXAMPLE 1

An aqueous protective coating composition was prepared by mixing thefollowing materials together in the following amounts.

Weight No. Ingredient Percent 1 Xanthan Gum (KELZAN ®) 0.051 2 Water1.698 3 Aqueous Colloidal Silica, 40% Silica 35.350 4 MicronizedMullite, 58% Al₂O₃, 40% SiO₂ (MJ5M) 37.740 5 Calcined Alumina, 6 microns(AC2-325M) 16.983 6 Metallic Non-wetting Agent 8.177 TOTAL 100.00

Broken down by ingredients, the aqueous protective coating compositioncontained the following:

No. Ingredient Weight Percent 1 Xanthan Gum (KELZAN ®) 0.051 2 Water22.908 3 Colloidal Silica 14.140 4 Silica from Mullite 15.096 5 Al₂O₃from Mullite 21.890 6 Impurities from Mullite 0.755 7 Calcined Alumina16.983 8 Metallic Non-wetting Agent 8.177 TOTAL 100.00

The aqueous protective coating composition can be provided as a two partsystem, with one part containing wet ingredients and the other partcontaining dry ingredients. In one embodiment, the aqueous protectivecoating composition can be prepared by first combining and mixing thesynthetic or natural gum (ingredient 1) with water (ingredient 2) toprovide a first mixture. The first mixture can be combined with theaqueous colloidal silica (40% silica) in a separate container using astirring blade for about 5 minutes, or the time needed to achievehomogeneity, to provide a second mixture. The mullite, calcined aluminaand non-wetting agent can then be slowly added to the second mixture andstirred for about 10 minutes, or a time needed to achieve homogeneity.The resulting aqueous protective coating composition can settle duringprolonged storage, and can be stirred again prior to use.

The aqueous protective coating composition can then be applied to aconcrete or masonry surface, such as a refractory wall or lining used tocontain molten aluminum in a furnace or vessel. After drying, theresulting dried protective coating has the following composition.

No. Ingredient Percent by Weight 1 Xanthan Gum 0.067 2 Colloidal Silica18.342 3 Silica from Mullite 19.582 4 Al₂O₃ from Mullite 28.421 5Impurities from Mullite 0.980 6 Calcined Alumina 22.031 7 MetallicNon-wetting Agent 10.607 TOTAL 100.00

EXAMPLE 2

An aqueous protective coating composition prepared according to Example1 was coated on one side of a 65% alumina refractory cup that had beenpre-filled to 1832° F. The other side of the cup was uncoated. The cupwas then filled with a 7075 molten aluminum alloy mixed with 2.5%magnesium and was maintained at a temperature of 1562° F. for 120 hours.The cup was then emptied and observed. The coated side of the cup showedno visual evidence of reaction with the molten metal. The uncoated sideof the cup showed substantial visual evidence of corrosion anddegradation.

EXAMPLE 3

Using the low cement castable (LCC) refractory cups, one cup was fullycoated with the aqueous refractory composition of Example 1 and theother cup was left uncoated. Both cups were filled with K₂CO₃, sealed,and heated to 1100° C. (2012° F.) for five hours in a sealed sagar.Then, the cups were emptied and inspected. The coated cup showed novisual evidence of reaction with the molten alkali metal. The uncoatedcup showed substantial visual evidence of corrosion and degradation.

The embodiments of the invention described herein are exemplary. Variousmodifications and improvements can be made without changing the spiritand scope of the invention. The scope of the invention is indicated bythe appended claims, and all changes that fall within the meaning andscope of equivalents are intended to be embraced therein.

We claim:
 1. A method of providing a protective coating compositioncomprising about 5% by weight to about 40% by weight water and about 60%to about 95% solids, comprising the steps of: a) combining and mixing aquantity of a water-soluble thickening agent with a quantity of water toprovide a first mixture; b) combining and mixing the first mixture withan aqueous colloidal silica dispersion to provide a second mixture; andc) adding Al₂O₃, SiO₂, and a non-metallic wetting agent to the secondmixture, and mixing the ingredients together to form the aqueousprotective coating composition.
 2. The method of claim 1, wherein thewater-soluble thickening agent comprises a synthetic or natural gum andconstitutes about 0.01% to about 2% by weight of the solids.
 3. Themethod of claim 1, wherein at least some of the Al₂O₃ and SiO₂ areprovided as mullite and the mullite constitutes about 30% to about 70%by weight of the solids.
 4. The method of claim 3, wherein the mullitecomprises about 55-60% by weight Al₂O₃, about 38-43% by weight SiO₂,less than about 1% by weight Fe₂O₃, about 1-2% by weight TiO₂, and lessthan about 1% by weight alkali and alkaline earth metal oxides.
 5. Themethod of claim 3, wherein the mullite is micronized to a medianparticle diameter of about 1-100 microns.
 6. The method of claim 1,wherein the colloidal silica constitutes about 10% to about 25% byweight of the solids.
 7. The method of claim 1, wherein at least some ofthe Al₂O₃ is provided as calcined alumina and the calcined aluminaconstitutes about 15% to about 35% by weight of the solids.
 8. Themethod of claim 1, wherein the metallic non-wetting agent constitutesabout 3% by weight to about 13% by weight of the solids.
 9. The methodof claim 1, wherein the metallic non-wetting agent is selected from thegroup consisting of metal phosphates, zirconium silicates, Group IImetal sulfates, Group II metal halides, metallic nitrides, andcombinations thereof.
 10. The method of claim 1, wherein the metallicnon-wetting agent is selected from the group consisting of magnesiumaluminate spinel, stack structures of alumina and zirconia, aluminumhalides, calcium aluminate, calcium hexaluminate, calcium silicate,aluminum titanate, and combinations thereof.
 11. The method of claim 1,wherein the metallic non-wetting agent is selected from the groupconsisting of Group II metal oxides; Group II metal celsians; fluorides,chlorides and bromides of titanium, zirconium, halfnium, copper andstrontium; and combinations thereof.
 12. The method of claim 1, whereinthe mullite, colloidal silica, calcined alumina and non-metallic wettingagent together constituted at least about 94% by weight of the solids.13. The method of claim 1, wherein the mullite, colloidal silica,calcined alumina, and non-metallic wetting agent together constitute atleast about 97% by weight of the solids.
 14. The method of claim 1,wherein the Al₂O₃ constitutes about 30% to about 70% by weight of thesolids and the SiO₂ constitutes about 25% to about 50% by weight of thesolids.
 15. A method of providing a protective coating compositioncomprising about 10% to about 30% by weight water and about 70% to about90% by weight solids, comprising the steps of: a) combining and mixing athickening agent with water to provide a first mixture; b) combining andmixing the first mixture with an aqueous colloidal silica dispersion toprovide a second mixture; and c) adding Al₂O₃, SiO₂, and a metallicnon-wetting agent to the second mixture, and mixing the ingredientstogether to form the aqueous protective coating composition; wherein theaqueous protective coating composition is substantially free of organiccomponents.
 16. The method of claim 15, wherein step c) comprises thestep of adding mullite having the formula 3Al₂O₃·2SiO₂, the mulliteproviding at least some of the Al₂O₃ and SiO₂ added in step c).
 17. Themethod of claim 15, wherein the Al₂O₃ constitutes about 30% to about 70%by weight of the solids, the SiO₂ (including colloidal silica)constitutes about 25% to about 50% by weight of the solids, and at leastsome of the Al₂O₃ is provided as calcined alumina.
 18. The method ofclaim 16, wherein the mullite comprised micronized mullite.
 19. Themethod of claim 15, further comprising the steps of applying theprotective coating composition to a refractory substrate and evaporatingthe water from the protective coating composition.
 20. The method ofclaim 15, further comprising the step of contacting the protectivecoating composition with molten aluminum or alkali metal vapor.
 21. Amethod of providing a protective coating composition, comprising thesteps of: applying an aqueous coating composition to a refractorysubstrate, the aqueous coating composition comprising water and solids,the solids including about 20% to about 90% by weight Al₂O₃, about 15 toabout 55% by weight SiO₂ and about 1% to about 15% by weight of ametallic non-wetting agent; evaporating the water from the aqueouscoating composition to yield the protective coating composition; andcontacting the protective coating with molten aluminum or alkali metalvapor, wherein the protective coating composition protects therefractory substitute from chemical reaction with the molten aluminum oralkali metal vapor.